1. Field of the Invention
The present invention relates to a flat or a semi-flat element including a partly or completely circumambient frame and an intermediate wall section.
2. Description of Related Art
Products made of plastic material are seen almost everywhere today. One way of manufacturing these products is through injection moulding of a thermoplastic material. It is a well known fact that the material thickness in such injection moulded products may not differ too much within the product since problems with warping will otherwise occur. This warping is caused by the temperature related shrinking, which is relatively great in most thermoplastic materials. An injection moulded product will normally be removed from the mould before it is completely cooled since the cycle time is an important economical factor in plastic manufacturing. The remainder of the cooling will hereby take place outside the mould. The shrinkage is related to the temperature which means that a part removed from the mould when hot will shrink more than a part removed when cold since some of the natural shrinkage can be counteracted by “freezing” the shape of the part in the mould. This means that thicker parts, which naturally contains more heat than thinner parts in the same product, will continue to shrink when cooling outside the mould. This will inevitably cause warping in the product. This problem has so far been prevented by designing the products with uniform material thickness.
There are however some cases where it would have been an advantage to be able to design products with different material thicknesses. One such example is containers where the walls and the bottom does not have to be particularly strong and the carrying structure of the container has to be strong since a lot of containers are to be stacked on top of each other, adding load to the container in the bottom of the stack. The thickness of the side walls and the bottom will in this case have to be over-dimensioned to be adapted to the injection moulding process since the carrying structure has to be sturdy. This will lead to a container heavier than necessary and that more material than necessary is used. This will of cause lead to an uneconomical product.